The nature of cellular and humoral in vitro reactions is being investigated in BALB/c mice implanted with syngeneic mammary tumors. Three different lines of this mouse strain are used: (1)\BALB/cCrgl, which has a low incidence of spontaneous mammary tumors (SMT) and is devoid of intact mouse mammary tumor virus (MMTV); (2)\BALB/cfC3H with a high incidence of SMT and expression of complete MMTV particles; and (3)\hereditarily asplenic BALB/c mice heterozygous for the nude gene, which have a high incidence of SMT appearing at an early age. Although endogenous MMTV genome is present in all tissues of these mice, it appears to be expressed preferentially in B lymphocytes. Different lymphoreticular cells appear to be stimulatable by MMTV antigens in the various colonies of BALB/c mice. The lymphocyte subset responding in blastogenesis to MMTV antigens is determined by the presence or absence of exogenous MMTV. Using purified tumor cell membranes from MMTV-positive mammary tumors, it has been found that some cell-mediated immunity assays are capable of reorganizing tumor associated (TAA) and viral antigens, while others can solely detect viral antigens. Methodology was devised to separate lymphoid cells present within the mammary tumors using centrifugal elutriation techniques. A study of the in situ lymphoreticular cells revealed that 95% of the tumor-infiltrating lymphocytes (TIL) are Thy 1+, and a high percentage of these cells expresses the Ly 2 marker. These TIL were found to be nonresponsive to TAA but they behaved as suppressor cells. Analysis of the underlying factors leading to the generation of these immunoregulatory cells will provide knowledge to bypass or overcome this suppressive activity. (LB)